In the field of tufting, there have been a variety of efforts made to enable both cut pile and loop pile tufts or bights of yarn to be placed in the same row of stitches. In some instances, the structures utilized for this purpose did not allow effective control of the height of stitches and, for instance, the cut pile stitches might always be of greater height than the loop pile stitches. The use of pivoting gate structures on the loopers was proposed in Jolley, U.S. Pat. No. 4,134,347 and Crumbliss, U.S. Pat. No. 4,353,317.
Later sliding gate structures were proposed as typified by Bennett, U.S. Pat. No. 6,155,187. When properly implemented, sliding gate structures may provide rapid response and avoid moving the entire pneumatic activation assembly with the loopers. However, Bennett taught the use of internal biasing elements in pneumatic cylinders and the use of blocks of cylinders to improve efficiencies in assembly. In practice, the use of internal biasing elements limits the size and corresponding force that the biasing elements may provide. In turn, this limits the speed with which the gate can return to the open position after pressure to its corresponding pneumatic cylinder is stopped. Furthermore, the internal biasing elements are not visible to inspection and if rust begins to form due to moisture in the cylinder, for instance, there will be no way to detect the problem until performance degrades to the point where defective carpet patterns are produced, with resulting waste carpet and the need to replace an entire cylinder block rather than merely a spring or biasing element.
A sliding gate structure utilizing an external spring was proposed in Kilgore, U.S. Pat. No. 7,222,576. Other efforts to improve the operation of gated loopers have focused on the gate assembly itself as in Johnston, U.S. Publication No. 2005/0109253.
The spring return gates suffer from a number of shortcomings, regardless of whether the spring is internally or externally placed. Principal among these shortcomings are the durability of the springs and the fact that a spring's biasing force changes over the range of compression of the spring. Thus, the durability of springs manifests itself over time as the spring material fatigues and the biasing force provided by the springs to slide the gate structures to the return position is diminished. Eventually, springs will even break from mechanical fatigue.
In addition, the further a spring is compressed, the greater the biasing force of the spring acting against the compression. Thus, if the spring is oriented to return the gate to retracted position, the spring is nearly fully compressed when the gate reaches its extended position. As the gate approaches the fully extended position, the spring is more fully compressed and the biasing force acting against the air pressure of a pneumatic cylinder increases. Due to friction between moving parts and the increased biasing force acting against the pneumatic pressure, some gates stick or fail to reach a fully extended position. Similarly, the further a spring is decompressed, the less biasing force the spring possesses. As the spring force gets weaker, it may fail to force all of the air in the cylinder to exhaust, causing the gate to stick before returning the gate to the fully retracted position.
It is desirable to address these shortcomings of spring biased gate structures without significantly increasing the cost or complexity of the gate control mechanisms.